1. Field of the Invention
The present invention relates to a linear actuator of the type which is based on a spindle.
2. The Prior Art
Actuators of this type are generally known and used. By way of example, reference may be made to WO 98/30816 A1 to Linak A/S, which relates to an actuator with an activation rod guided in an outer tube, and DK 174 457 B1 to Linak A/S, in which the spindle nut is intended for direct attachment to the application. An example of an application is found, e.g., in EP 488 552 A1 to Huntleigh Technology plc, which relates to a hospital bed with several actuators coupled to a common control box containing a power supply and a control. A special embodiment of actuators for domestic beds with two spindles, an integrated power supply and control is known from EP 372 032 B2 to Dewert Antriebs- and Systemtechnik GmbH & Co. KG.
A spindle with an associated spindle nut is a central element of these actuators. In case of spindles of a non-self-locking type, they are provided with a brake, or, alternatively, a transmission with a low efficiency is selected, so that the transmission is self-locking. A perfect operation of these elements is critical to the operation of the actuator. A typical transmission is a worm gear which is of plastics owing to noise, but on the other hand it is more exposed to wear than a worm gear of metal. In order to stop the activation rod of the actuator, the actuator is provided with end stops. It may be end stop switches which are activated by the spindle nut or an outer pipe. Another form of end stops is a mechanical stop for the spindle nut. Still another form of end stops is based on optical or magnetic encoders which count the revolutions of the spindle. When the nut engages the mechanical stop, the motor current is increased considerably and is interrupted by the control electronics. Again, a safe function of these elements is conditional upon a perfect function of the spindle and the spindle nut. Indirect damage may also occur because of wear of the seal between the activation rod and the outer end of the surrounding protective tube. Penetrating dust and moisture also act to wear out the spindle and the spindle nut, just as end stops and other components are likewise exposed to wear-out and malfunction.
To prevent damage to individuals and equipment, the actuators may be equipped with various safety measures, such as safety nut, spline function, electrical overload protection, etc.
An example of a safety-critical application is a patient lifter which acts like a crane and can lift patients out of beds, chairs, etc. In such an application, it is critical that the actuator does not fail suddenly.
The manufacturers of actuators guarantee a certain service life against compliance with mounting specifications and service instructions. However, it has been found that the actuators are used far beyond the guaranteed service life, where the risk of failure is the greater the longer they are used. Nor is it unusual that the actuators are used for greater loads than the pre-scribed maximum load, or in an environment for which they are not intended.
In the event of an unfortunate occurrence or direct accidents, it may be difficult to elucidate the concrete conditions of the actuator which have caused the event. All that one has to go by is various marks which make it possible to recover the product specifications and the production time at the manufacturer's. First and foremost, it is a matter of avoiding unfortunate occurrences and accidents, but to achieve this it is decisive that the manufacturers get an adequate basis for improving the quality and the safety of the actuators. In the second place, allocation of responsibility is also involved, which, in turn, influences the insurance conditions and contracts.
In this connection it is observed that the cost level plays a decisive role. If the selling price is too high, it is seen that safety measures having no immediate effect are not adopted. It is noted that for cost reasons the actuators are constructed such that they can momentarily accept a certain overload, a constraint in the form of an intermittence, typically 10%, being simultaneously imposed, which means that an actuator may only run for 6 minutes out of an hour.
The object of the invention is to provide an actuator where a more adequate basis for evaluating the state of the actuator may be obtained.